This invention generally relates to drill jigs, and is specifically concerned with a drill jig that utilizes a hydraulic chuck in combination with a drill guide having a pilot bore to accurately drill holes to tight tolerances.
Jigs for guiding drills to perform precision boring operations are well known in the prior art. Such jigs are frequently used in the manufacture of commercial jetliners where the assembly of a single wing requires the boring of 1.5 million holes through various aluminum and titanium components. These prior art jigs are used in combination with precision made fixtures to accurately place the tip end of a drill along the center line of a desired bore location, and then to extend and retract a rotating drill in order to create a bore of the desired diameter in concentric alignment with the desired location.
One of the drill jigs presently used in the manufacture of airliner wings comprises a rotatable spindle connected on one end to the output shaft of an air motor, and to the shank of a drill at the other end. The spindle is concentrically arranged within a housing tube (or nosepiece as it is known in the art). The housing tube is stationarily coupled to the motor casing. To provide a means for advancing and retracting the drill, a screw thread is provided around the exterior of the spindle. The screw thread on the spindle is engaged to a screw thread provided around the inner diameter of a gear driven by the motor.
In operation, a fixture connected to the housing tube moves the jig into precise coaxial alignment with the center line of the situs of the desired bore. Next, the air motor is actuated to rotate the drill at the desired rpm. The pitch of the screw thread around the spindle is dimensioned such that the drill advances at the desired feed rate per revolution, which is slow enough for the cutting edges on the tip end of the drill to cleanly cut and remove material through the aluminum or titanium workpiece. Once the bore has reached the desired depth, the air motor is stopped and reversed so that the screw thread on the spindle acts to withdraw the drill from the hole.
Unfortunately, such a jig structure has proven inadequate for the production of the high-precision bores required in the manufacture of the flap-track of a commercial jetliner. Such bores must be drilled to a very tight tolerance, with a run-out (i.e., slight conical tapering of the bore due to off-center wobbling of the drill) of only plus or minus 0.004 inches. Because such jigs cannot initially produce a bore to within such tight tolerances, the bores must be drilled under-sized, and then precision enlarged by way of a special reaming operation to bring them within the required tolerances. The production of such precision bores is essential in such aeronautical components, as such components are subjected to constant vibration in use, which can cause premature metal fatigue if the components forming the wings and other parts of the aircraft are not precision-fitted. However, while the drilling step takes only a short time, the additionally-required precision reaming step is many times longer, thus substantially increasing the manufacturing time for the production of each precision bore around the flap track. Still another shortcoming associated with the prior art jig design is that it is compatible for use with only specially manufactured drills that have a shrink-fitted Morse taper at the end of their shanks for coupling with the threaded end of the spindle. The installation of a Morse taper and machining of the bore for the screw-type coupling not only creates opportunities for unwanted non-concentricities to form between the spindle and the drill, but renders the changing of a worn drill bit with a new one an awkward and time consuming operation, as much of the jig must be disassembled.
Clearly, what is needed is a drill jig compatible for use with an automated fixture which is capable of drilling bores to tight tolerances and with minimal run-out without the need for additional, time-consuming reaming steps. Ideally, such a jig should also provide for the easy and rapid changing of a worn drill with a new one. Finally, such a jig should be compatible for use with integrally formed drills that do not require the installation of Morse tapers or special coupling components.